In recent years, wireless Local Area Networks (LANs) have become a significant technology in enterprise networks, public networks and home networks. Their high data rates and convenience of use enable the deployment of increasingly powerful (and bandwidth-hungry) mobile computing and communications devices. As a result, the use of wireless LANs, and the proliferation of devices adapted for operation in such networks, continues to accelerate.
At present, there are various methods and protocols for enabling communications between devices participating in a network, offering various levels of reliability, robustness and effectiveness. Similarly, a number of algorithms and methods have been developed to extend various network services to these network devices. These protocols typically work within the structure defined by the Open Systems Interconnection (OSI) reference model promulgated in 1984 by the International Organization for Standardization (ISO).
As depicted schematically in the block diagram of FIG. 1A, the OSI reference model includes seven layers of network services including the Application layer 24 (the “highest” layer), the Presentation layer 22 below the Application layer, then the Session layer 20, the Transport layer 18, the Network layer 16, the Data Link layer 14, and finally the Physical layer 12 (the “lowest” layer).
Within the reference model, the Data Link layer offers various services to the Network layer, principally, transferring data from the Network layer of a source device to the Network layer on the destination or target device. The typical method is for the Data Link layer to break up the bit stream into discrete blocks of bits, compute a checksum for each block, and transmit the block along with the checksum to the target device in the form of a packet. When a packet arrives at the target device, the checksum is recomputed for the received block. If the newly computed checksum is different from the checksum provided by the source device, the Data Link layer identifies that an error has occurred and an error-recovery process is invoked.
At the Medium Access Control (MAC) sublayer of the Data Link layer, protocols are used to solve the issue of which network device gets to use the broadcast channel when there is competition for it. The MAC layer is particularly significant in LANs, in which the number of network devices competing for the communications channel may be very large.
Various examples of MAC protocols are set forth in U.S. Pat. Nos. 6,597,683; 6,621,872; 6,532,225; 6,546,001; 5,371,734; and 6,590,890.
One commonly used MAC technique is Carrier Sense Multiple Access (CSMA), in which a transmitting device first listens to the communications channel (carrier sensing) to determine if another device is transmitting at that moment.
Various versions of conventional CSMA are in use. In one, known as 1-persistent CSMA, a device that wishes to transmit executes carrier sensing and if the channel is busy, the device waits until the channel becomes idle. When the device detects an idle channel, it transmits its packet. Collisions can still occur, since a channel may be idle at the exact moment of carrier sensing, yet devices may be about to transmit their packets at about the same time.
In another CSMA version, nonpersistent CSMA, if carrier sensing indicates that the channel is in use, the device will wait for a random period of time and then start to transmit.
In current, conventional wireless networks, there are three specifications in the IEEE Wireless LAN (WLAN) 802.11 family based on different Physical layer (reference numeral 12 of FIG. 1A) technologies: 802.11, 802.11a, and 802.11b. The general features of these specifications are well known in the art. All three of these specifications use CSMA/Collision Avoidance (CSMA/CA), also known as the distributed coordination function (DCF).
As depicted in FIG. 3 (Prior Art), in accordance with DCF, a node that attempts to access the channel first performs carrier sense (302) to determine the status of the medium. If the medium is busy (304), the node will defer (306) until the medium is idle for a period of time equal to DIFS (DCF Inter-Frame Space). After this DIFS idle period, the node generates a random back-off counter (308), which corresponds to the number of idle timeslots this node has to wait additionally before its transmission. The back-off counter is uniformly distributed over the interval [0, CW], where CW is the current Contention Window size.
The back-off counter is decremented by 1 after each idle slot (310). The back-off counter will be suspended once the medium becomes busy (312), and will be decremented when the medium is idle again for DIFS duration (314). Once the back-off counter reaches zero (316), the node is free to transmit. If more than one node starts to transmit in the same slot, a collision occurs (318). Upon collision, the colliding packets may be retransmitted until the number of retransmissions reaches the retry limit or a successful transmission is made (320).
Besides the basic access scheme presented above, DCF also has an optional virtual carrier sense scheme based on RTS/CTS (request to send/clear to send) frame exchange. The RTS/CTS access mode exchanges RTS/CTS handshake frames before the actual transmission of data packet to perform fast collision detection and channel reservation for large data frames. By using RTS/CTS handshake, each node in the neighborhood maintains a prediction of future traffic on the medium for the forthcoming data packet.
The CSMA/CA protocol has proven reasonably effective for applications with relatively low traffic load. As traffic loads increase, however, the effectiveness of the protocol declines, and its drawbacks become more pronounced. Moreover, as the use of wireless LANs continues to increase at near-exponential rates, future users will want the capability to communicate anywhere, anytime, from any device, with any content. Such communication needs require energy and bandwidth efficiency as well as quality of service (QoS) capability at the MAC layer—characteristics which CSMA/CA and other conventional protocols do not provide.
In the face of these challenges for future multimedia wireless packet networks, an improved, distributed media access protocol is required.
In particular, it would be desirable to provide a protocol that significantly improves communications efficiency and provides substantial savings in communications power.
It would also be desirable to provide a protocol that improves achievable network capacity so as to accommodate a larger number of users.
It would be desirable to provide a protocol that provides fair channel sharing among an arbitrary number of users, with fast convergence.
It would also be desirable to provide a protocol that enables inherent Quality of Service (QoS) capability with quantitative performance guarantees for different priority users.
Still further, it would be desirable to provide a protocol that is simple to implement, with low computational complexity and no additional control overhead or buffer requirements.